Abstract Aberrant proliferation of epithelial stem cells in the adult prostate can contribute to hyperplastic growth (e.g., benign prostatic hyperplasia, BPH) and subsequent quality of life challenges. Despite intensive study, little is known about the prostate epithelial stem cell hierarchy and resulting lineage relationships. Bona fide epithelial stem cells are defined by their self-renewal, multipotency, and regenerative capabilities. Additionally, it is thought that prostatic stem cells can survive without androgen signaling, in contrast to differentiated cells. The mentor's lab has previously shown that the transcription factor Sox2, well known for its roles in maintenance of embryonic stemness, is expressed in adult murine and human prostate epithelium. Further, unpublished data from the mentor's lab indicate that Sox2+ cells are formed just prior to embryonic development of the prostate and can persist after androgen deprivation. Using a genetic lineage tracing strategy, we have observed that Sox2-expressing cells of the prostate can persist long- term (over 3 months), suggesting that they may be progenitor cells. Defining the contribution of Sox2+ cells to tissue homeostasis and regeneration is essential to understanding the prostate stem cell lineage hierarchy, a current gap in knowledge in the field. These data are critical to development of novel strategies that target progenitor cell populations in BPH, an unmet therapeutic need. However, it remains unclear whether Sox2+ cells are bona fide stem cells and/or are required for tissue development and homeostasis. The scientific objective of this application is therefore to bridge this knowledge gap by studying the contributions of Sox2+ cells to the homeostasis of prostate epithelium in health and disease. The proposal is innovative because it will be the first to determine whether Sox2 expression maintains adult prostate stem cells, which has not been previously investigated. The proposed research is significant because it will link specific regulatory mechanisms of stem cell maintenance to benign prostatic hyperplasia, allowing therapeutic exploitation of fundamental cellular processes. In addition to benefitting diseases associated with prostatic hyperplasia, the concepts and tools developed may be applicable to the study of epithelial stem cells for diseases of other organs that are driven by hormonal dysfunction.